The construction cost of fine roots was studied in 23 woody species and two grassland communities, growing under natural conditions in southern Spain. Calculation of the energy (glucose) required for their synthesis was based on the quantification of chemical components present in tissues. Despite considerable differences in the chemical composition of the three life forms studied (trees, shrubs and herbaceous), detected differences in construction cost were non-significant (mean value: 1·64 ± ± ± ± 0·13 g glucose g− 1 ). However, shrubs and herbaceous plants growing in more fertile habitats expended significantly less energy on root synthesis (1·58 ± ± ± ± 0·06 and 1·41 ± ± ± ± 0·05 g glucose g− 1 , respectively) than those growing in less fertile areas (1·80 ± ± ± ± 0·06 and 1·57 ± ± ± ± 0·1 g glucose g− 1 , respectively), because they contained smaller amounts of either waxes (shrubs) or lignins (herbaceous), both expensive to synthesize, and, proportionately, more cellulose; which is inexpensive to synthesize. Deciduous and evergreen tree species also differed mainly with regard to wax and cellulose contents, giving rise to a significantly higher construction cost in evergreens (1·57 ± ± ± ± 0·07 g glucose g− 1 versus 1·78 ± ± ± ± 0·02 g glucose g − − − − 1 ). The differences observed in construction cost appeared to be due more to habitat-induced differences in chemical composition than to any intrinsic difference between the species studied.

We tested whether growth and maintenance costs of plant organs vary with environmental stress. Quercus ilex L. seedlings from acorns collected from natural populations in the northern Iberian Peninsula and in a lower elevation and putatively less stressful habitat in the southern Iberian Peninsula were grown in pots under the same conditions. Growth and maintenance respiration were measured by CO(2) exchange. Young leaves from 5-month-old seedlings of both populations had similar mean specific leaf areas, nitrogen and carbon concentrations and specific growth rates, and almost identical growth costs (1.26 g glucose g(-1)). Leaf maintenance cost was higher in northern than in the southern population (27.3 versus 22.4 mg glucose g(-1) day(-1), P < 0.01). In both populations, leaf maintenance cost decreased by 90% as leaves aged, but even in mature leaves, the maintenance cost was higher in the northern population than in the southern population (3.38 versus 2.53 mg glucose g(-1) day(-1), P < 0.01). The growth costs of fine roots < 1 mm in diameter were similar in the two populations (1.20 g glucose g(-1)), whereas fine root maintenance cost was higher in the northern population than in the southern population (9.86 versus 7.45 mg glucose g(-1) day(-1); P < 0.05). The results suggest that the cost of organ maintenance is related to the severity of environmental stress in the native habitat. Because the observed differences in both leaves and roots were constitutive, the two populations may be considered ecotypes.

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Root construction and maintenance costs were estimated in four evergreen and three deciduous Quercus species that are typical in the landscape of southern Spain. The cost quantification was based on analysis of the growthrespiration ratio. Values observed for both construction cost (ranging from 1·17 to 1·29 g glucose g − − − − 1 dry weight) and maintenance cost (ranging from 6·22 to 11·71 mg glucose g − − − − 1 dry weight d − − − − 1 ) were generally lower than those reported in other studies. The results showed non-significant differences between deciduous and evergreen species. The lack of significant differences between species appeared to be due to the homogeneity of growth conditions. Hydroponic culture, with unrestricted nutrient and water supply, would lead to low tissue carbon content and low respiration rates, leading to the low costs observed. Furthermore, the fact that root organs are clearly importers of organic molecules inevitably entails some underestimation of the respiration associated with growth and, to a lesser extent, with maintenance respiration. This leads in turn to underestimation of the corresponding construction and maintenance costs. All this raises doubts as to the suitability of this method for studying root systems.